Passivity and pseudopassivity of cemented carbides

During corrosion of cemented carbides the binder phase is selectively dissolved from the material. After the binder has been dissolved into the solution the remaining tungsten carbide skeleton can not withstand a mechanical force any longer and these loose grains can be easily removed even by light abrasion. In principle Co-based cemented carbide does not passivate. However the potentiodynamic curve of the material shows a conventional anodic behavior. It exhibits critical potential and breakthrough potential. Since the current density of WC-Co cemented carbides after reaching critical potential is several orders of magnitude higher when compared to true passivity this phenomenon is called pseudopassive behavior. Some precipitations could be formed in pseudopassive region at the interface of intact cemented carbide and the skeleton but they are not stable and re-dissolve into the corrosive solution. Modification of the binder with chromium carbide addition improves corrosion resistance of cemented carbides significantly. Chromium acts as an alloying element and yields to a formation of a mixed Co-Cr oxide layer as shown by EF-TEM analysis. This oxide layer is considered as a true passive layer decreasing the rate of dissolution of the binder. Vanadium has been found to dissolve into the binder besides of forming thin layer between binder and WC grains. Vanadium in the binder also improves corrosion resistance of cemented carbide.